Foundry molds and cores and process for making same



June 30, 1964 R. s. AMALA 3,138,836

FOUNDRY MOLDS AND CORES AND PROCESS FOR MAKING SAME Filed Dec. 27, 1960 TENSILE STRENGTH-PSI .5 L0 0.5 MOLE RATIO-FORMALDEHYDE UREA TENSILE STRENGTH- PSI l l I l l I 1 l l L0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 a H20 av WEIGHT or MOLD mx ATTORNEY United States Patent FOUNDRY MOLDS AND CORES AND PROCESS FOR MAKING SAME Raymond S. Amala, Oak Park, Mich, assignor to General Motors Corporation, Detroit, Mich, a corporation of Delaware Filed Dec. 27, 1960, Ser. No. 78,653 5 Claims. (Cl. 22193) This invention relates to foundry molds and cores for use in metal casting operations and more particularly to dispensable molds and cores having a resin binder.

Among the techniques presently employed in foundry practice for the production of precision or semi-precision castings are those in which resinous binders are used to hold together the sand particles of which the molds or cores are made. In general, these molds and cores involve the use of thermosetting binders such as phenolformaldehyde resin for the sand grains to form thinwalled molds or cores having a high gas permeability, good surface smoothness and dimensional stability. The molding material generally consists of a dry mixture of a major proportion of sand and a minor proportion of a thermosetting binder used in powdered form or as a coatiru of the sand particles. In the case of phenol-form aldehyde and similar synthetic resin binders, the resin is used in novolak form together with a curing agent such as hexamethylenetetramine or in the form of a resole.

Molds are prepared by allowing the dry molding mixture to come into contact with a hot pattern heated to a temperature on the order of about 350 F. to 600 for a time ranging from a few seconds to about a minute until a desired mold thickness is built up. Thereafter, the excess molding material is removed and the closely adhering sand-resin layer is cured by heating the mold while in contact with the metal pattern at a temperature ranging from 300 F. to as high as 1500 F. for a time sufficient to cure the resin into a hard and infusible material.

While the molds and cores manufactured in the manner described above are satisfactory for the casting of ferrous metals, the phenol-formaldehyde-type binders do not burn out and disintegrate with sufficient ease and thoroughness when casting a relatively low temperature metal such as aluminum with the result that shake-out problems are greatly magnified.

It is the basic object of this invention to provide an improved and more eiiicient method for making resin bonded molds and cores. It is a further object of this invention to provide economical shell-type molds and cores in which the sand is held together by means of an inexpensive binder which will readily withstand the force of the molten metal during the casting process and which will readily burn out and disintegrate in the process of casting relatively low casting temperature metals such as aluminum and the alloys thereof.

These and other objects of the invention are accomplished by first preparing a sand-binder molding mixture which includes a major proportion of sand admixed with a minor proportion of a binder material which consists essentially of an aqueous solution of formaldehyde in admixture with urea in which the formaldehyde and the urea are present initially as separate compounds in a proportion of about 2 to 2.6 moles of formaldehylde to one mole of urea and the water is present in amounts ranging from about 1.5% to about 3.5% of the weight of' the sand.

In accordance with another object of the invention, a small quantity of sublimating chemical such as naphthalene, chlorinated naphthalene or camphor is incorporated in the foundry sand to improve the flow properties and lubricity thereof.

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Other objects and advantages will be apparent from the following detailed description of the invention, reference being had to the accompanying drawing, in which:

FIGURE 1 is a graph showing the effect of the mole ratio of formaldehyde to urea on the tensile strength of molds and cores made in accordance with this invention; and

FIGURE 2 is a graph showing the effect of the water content of the molding mixture on the tensile strength of molds and cores made in accordance with this invention.

It has been previously proposed to use urea-formaldehyde resins as a binder in molds and cores. However, heretofore insofar as it is known, urea-formaldehyde resins have not been successfully adapted for volume production use in the manufacture of molds and cores on a commercial basis. It is believed that among the important reasons for this is that the typical powdered molding urea-formaldehyde resins do not melt and flow sufficiently on being heated during the curing process to permit the satisfactory bonding of the sand grains and the successful manufacture of shell-type molds and cores. In contrast, successfully used resin binders such as phenol-formaldehyde resin in novolak form have the property of melting and flowing on being heated to elevated temperatures of about 350 F. When a molding mixture involving this type of resin is blown onto a hot pattern, the phenol-formaldehyde resin melts and flows so as to coat a plurality of adjacent sand particles whereby the sand particles become firmly bonded to one another. Typical heat-curable adhesive-type urea-formaldehyde materials are of a relatively viscous and syrupy character which makes the mixing and handling of sand-resin mixtures of this type a messy and diiiicult matter. The present invention involves a process by which these and other ditiiculties which have previously discouraged the use of urea-formaldehyde resins have been overcome and shell molds and cores of satisfactory high strength are obtained.

A specific example of the process and a resulting core is as follows. 3000 pounds of foundry lake sand are dumped'into a muller together with 45 pounds of corn flour and approximately pounds of water. The use of the corn flour is optional but is preferred to give the core green strength if, as in the instant illustration, the core is to be handled before it is cured. After thoroughly mixing these ingredients in the muller, the binder constituent is added preferably in a proportion of approximately 1% of the weight of the sand-flour mixture. To this end, 12 pounds of formaldehyde, 10 pounds of urea and 8 pounds of water are mixed together and added to the moist sand-flour mixture in the muller. The binder and sand-flour mixture together with the water contained therein are then thoroughly mixed together. The resulting water content is about 2.75% of the sand. After the binder has been thoroughly mixed into the sand, about 6 pounds of a sand conditioning agent or lubricant is added to the mixture which consists of a mixture of about kerosene and 20% naphthalene. Preferably, the binder component is mulled with the sand about three minutes and the lubricant is added during the last 30 seconds of the mixing operation.

The sand-binder mixture is now ready for use in the manufacture of a core. To this end, the mixture is preferably transferred into a core blower which includes a cold core box. The moist sand-binder mixture is then blown into the core box in a manner well known in the art. The core box is then opened and the green core is removed from the box. It is then placed in an oven for about 40 minutes maintained at about 350 F. to convert the urea and the formaldehyde into a hard and infusible resinous bonding agent. The baking time may 3 vary from 15 to 40 minutes or more depending upon the thickness of the core and the baking temperature which may vary between 350 F. and 500 F.

As shown in FIGURE 1, the relationship between the urea, the formaldehyde and water is sharply critical if it is desired to produce a core or mold having a satisfactory tensile strength for casting ferrous metals and light metals such as aluminum and magnesium. As shown in this graph, an optimum tensile strength is obtained when the ratio of formaldehyde to urea is about 2.3 moles. Usable cores or shell molds having a tensile strength of 200 or more p.s.i. may be obtained from molding compositions in which the ratio of formaldehyde to urea varies from about 2.0 to about 2.6.

As shown in FIGURE 2, the water content of the molding mixture is likewise critical to the production of satisfactory molds and cores. As this graph indicates, the total water content of the sand-resin mix must be at least 1.5% to produce cores having a tensile strength of 200 p.s.i. or more. A water content in excess of 3.5% produces molding compositions which are too wet for eflicient handling and cannot be used as a practical matter. Optimum handling ease and tensile strength is obtained when the water content is between about 2% and 3%. The optimum water content within these ranges will to some extent depend on whether or not a green strength additive such as cereal flour is used. These additives are generally of considerably finer mesh than the usual foundry sands and sand compositions including such additives will require a somewhat greater water content to properly wet the additive material. For example, in the instant illustration which contains 45 pounds of corn flour per 3000 pounds of sand, an optimum water content is about 2.75% by weight of the total mixture. When the corn flour is omitted, the optimum water content is reduced to about 2% by weight of the total mixture.

The presence of the water and the quantity thereof in the molding mixture is as indicated above, an important aspect of this invention. The water serves as a vehicle for the individual formaldehyde and urea compounds. It serves to distribute these constituents uniformly and thoroughly throughout the molding mixture with the result that when heat or a catalytic agent is applied to the mixture, the constituents react to form a urea-formaldehyde type resin material which efliciently binds the sand particles to one another.

In the preparation of molds or cores using the aforementioned foundry mixture, an acid salt such as ammonium chloride, ammonium sulfate or oxalic acid may be incorporated in the molding mixture in proportions up to about 1% to of the combined weight of the urea and formaldehyde and the resulting mixture may be blown directly into a hot box maintained at a temperature of about 400 F. to 600 F. The acid salt will serve the function of catalyzing the urea and formaldehyde mixture in the water to form a relatively hard, infusible urea-formaldehyde-type resin binding the sand particles together within seconds after the blowing operation is completed.

It will be observed that in the method of this invention no processing of the binder constituents is required prior to their use. The high bonding efficiency of the resulting resin permits the use of a relatively low percentage of between about 0.5% to 3% by weight based on the weight of the sand and preferably between about 0.75% and 1.5%.

The molding mixture of this invention may, as indicated above, include minor proportions of green strength providing additions. These include materials such as cereal flours, bentonite clays, silica flour and fly ash. For example, based on the weight of the sand, 0.5% to 2% ,by weight of cereal flour may be used, 0.5% to 6% bentonite clay may be used and up to 10% of 200 mesh silica flour may be used. The term sand" as used in the appended claims is intended to include the presence of green strength additives in nominal proportions.

As indicated above, a sublimating chemical such as naphthalene, chlorinated naphthalene or camphor may be advantageously added to the sand-resin mixture to improve the fiow properties and lubricity thereof. In the practice of manufacturing cores to be used for metal casting, it is frequently necessary to employ lubricants or mold release agents in the core mixture to prevent the sticking of the core sand to the core box and to improve the flow properties of the molding mixture. It has been found that in the molding mixture of this invention the class of chemical compounds which will sublime at or near room temperature will function satisfactorily as mold release agents. It is believed that the aforementioned organic compounds function well as lubricants and mold release agents principally because of their ability to sublime, their chemical inertness to the core binders and their inherent lubricating qualities. The sublimating characteristic of these compounds is of particular importance in the instant invention since gaseous products formed are condensed or are absorbed on the surface of the core box and the auxiliary pattern equipment thus providing greater lubricating or releasing efficiency. In practice, the mold release agents of this invention improve the blowability of the core mix, eliminate sand sticking in the core box and prevent resin build up on the patterns or core boxes. Improved results may be obtained by using as little as 0.02% of the mold release agent and satisfactory results are obtained when using up to 2% of the mold release agent. The release agents as indicated above may be added directly to the core mix either as the chemical itself or they may be added to the core mix as a solution of a petroleum distillate such as kerosene and the pure chemical. A composition consisting of 20% naphthalene and kerosene has been found to produce satisfactory results. This solution may be used in amounts ranging from about 0.1% to 0.7% based on the weight of the core mixture.

Although the invention has been described in terms of specific embodiments, it will be understood that other forms may be adopted within the spirit of the invention.

I claim:

1. A molding mixture for use in making foundry molds and cores comprising sand, a binder and a mold release agent, said binder consisting substantially of an aqueous solution of formaldehyde and urea, the proportion of said formaldehyde to said urea being in a mole ratio of about 2 to 2.6 and the proportion of said water being about 1.5% to 3.5% by weight of the weight of said mixture, said urea and said formaldehyde together being present in proportions of about 0.5% to 3.0% by weight, said mold release agent comprising a sublimating compound taken from the group consisting of naphthalene, chlorinated naphthalene and camphor in amounts of about 0.02% to 2.0% of said mixture by weight.

2. A molding mixture for use in making foundry molds and cores comprising sand, a binder and a mold release agent, said binder consisting substantially of an aqueous solution of formaldehyde and urea, the proportions of said formaldehyde to said urea being in a mole ratio of about '2 to 2.6, the proportion of the water being about 1.5 to 3.5% by weight of said mixture, said urea and said formaldehyde together being present in proportions of about 0.5 to 3.0% by weight, said mold release agent comprising naphthalene in amounts of about 0.02 to 2.0% of said mixture by weight.

3. A process for manufacturing shell-type molds and cores comprising the steps of admixing a major proportion of foundry sand with minor proportions of a binder and a mold release agent, said binder comprising an aqueous solution of formaldehyde and urea, said formaldehyde and said urea being present in proportions of about 2 to 2.6% moles of formaldehyde in the urea, the water being present in proportions of about 1.5 to about 3.5%

by Weight based on the Weight of said mixture, said mold release agent comprising a sublimating compound taken from the group consisting of naphthalene, chlorinated naphthalene, and camphor in amounts of about 0.02 to 2.0% of said mixture by weight, molding said article from said molding mixture, and subjecting said molded material to heat at a temperature range from about 350 F. to 600 F. for time sufficient to cause said formaldehyde to form an infusible bond between said particles.

4. A molding mixture for use in making foundry molds and cores comprising a major proportion of sand and minor proportions of a binder and a mold release agent, said binder consisting substantially of an aqueous solution of formaldehyde and urea, the proportions of said formaldehyde to said urea being in a mole ratio of about 2 to 2.6, the proportion of the water being about 1.5 to 3.5% by Weight of said mixture, said mold release agent comprising a sublimating compound taken from the group consisting of naphthalene, chlorinated-naphthalene and camphor in amounts of about 0.02% to 2.0% of said mixture by weight.

5. A mold component comprising a major proportion of sand and a minor proportion of a urea-formaldehyde binder, said mold component being formed from a molding mixture including sand, said binder and a mold release agent in which the sand is wetted with the binder consisting substantially of an aqueous solution of formaldehyde and urea, the proportion of said formaldehyde to said urea being in a ratio of about 2 to- 2.6 and the proportion of said Water being about 1.5 to 3.5% by weight of said mixture, and said mold release agent is a sublimating compound taken from the group consisting of naphthalene, chlorinated naphthalene and camphor in amounts of about 0.02 to 2.0% of said mixture by Weight.

References Qited in the file of this patent UNITED STATES PATENTS 2,422,118 Meyer June 10, 1947 2,534,806 Webber et a1 Dec. 19, 1950 2,683,296 Drumm et a1 July 13, 1954 2,818,397 Casebolt Dec. 31, 1957 2,869,195 Cooper et al. Jan. 20, 1959 3,008,205 Blaies Nov. 14, 1961 

1. A MOLDING MIXTURE FOR USE IN MAKING FOUNDRY MOLDS AND CORES COMPRISING SAND, A BINDER AND A MOLD RELEASE AGENT, SAID BINDER CONSISTING SUBSTANTIALLY OF AN AQUEOUS SOLUTION OF FORMALDEHYDE AND UREA, THE PROPORTION OF SAID FORMALDYHYDE TO SAID UREA BEING IN A MOLE RATIO OF ABOUT 2 TO 2.6 AND THE PROPORTION OF SAID WATER BEING ABOUT 1.5% TO 3.5% BY WEIGHT OF THE WEIGHT OF SAID MIXTURE, SAID UREA AND SAID FORMALDEHYDE TOGETHER BEING PRESENT IN PROPORTIONS OF ABOUT 0.5% TO 3.0% BY WEIGHT, SAID MOLD RELEASE AGENT COMPRISING A SUBLIMATING COMPOUND TAKEN FROM THE GROUP CONSISTING OF NAPHTHALENE, CHLORINATED NAPHTHALENE AND COMPHOR IN AMOUNTS OF ABOUT 0.02% TO 2.0% OF SAID MIXTURE BY WEIGHT. 